Process for synthesizing urea

ABSTRACT

In the process for urea synthesis, an effluent from a reactor 2 containing urea, ammonium carbamate, excess ammonia and water is introduced in a first heater 3 to separate excess ammonia and to decompose ammonium carbamate, thereby separating them from the liquid phase; then the liquid effluent from the first heater 3 is introduced in a second heater 6 to separate the remaining excess ammonia and to decompose ammonium carbamate in the presence of starting carbon dioxide used as stripping gas, thereby separating them from the liquid phase; the liquid effluent from the second heater 6 is introduced into a urea solution purification step in which pressure is lower than in the above step; and the separated gaseous effluents from the first and the second heaters are returned into the reactor.

FIELD OF THE INVENTION

The present invention relates to a process for synthesizing urea.

More particularly, the present invention relates to a process whereinthe production cost of urea is reduced by efficiently returningreactants not converted to urea in the urea synthesis reaction into thereactor for the urea synthesis.

DESCRIPTION OF THE PRIOR ART

In the production of urea, the starting materials, i.e., ammonia andcarbon dioxide, are substantially quantitatively converted to ammoniumcarbamate under usual reaction conditions, for example, comprising apressure of 200 Kg/cm² and a temperature of 190° C., in the presence ofexcess ammonia, as shown in reaction formula (1). Ammonium carbamate isfurther converted to urea as shown in reaction formula (2) wherein theequilibrium conversion is determined by the reaction conditions and theactual conversion is determined by the rate of attaining the equilibriumconversion, which rate depends on the residence time in the reactionzone:

    2NH.sub.3 +CO.sub.2 ⃡NH.sub.2 COONH.sub.4      ( 1)

    NH.sub.2 COONH.sub.4 ⃡NH.sub.2 CONH.sub.2 +H.sub.2 O (2)

The actual conversion is about 50-75% industrially.

Namely, the excess ammonia and about 25-50%, based on the theoreticalamounts, of the starting materials ammonia and carbon dioxide remainunconverted to urea.

A known process for separating urea from unconverted materials comprisesheating an effluent from the reaction zone under reduced pressure todecompose ammonium carbamate into ammonia and carbon dioxide, therebyseparating them in the form of a gas together with the excess ammonia,absorbing the ammonia and carbon dioxide in a recovered liquid formed inthe subsequent step of decomposing and absorbing ammonium carbamateunder a lower pressure, and returning the recovered liquid into thereaction zone.

Another known process comprises stripping an effluent from the reactionzone by subjecting it to gas-liquid contact with gaseous startingmaterials, ammonia or carbon dioxide, under a pressure which issubstantially not reduced, thereby decomposing and separatingunconverted ammonium carbamate and returning the decomposition productsinto the reaction zone. This process utilizes the fact that an effectequivalent to that obtained by pressure reduction is obtained by thegas-liquid contact.

SUMMARY OF THE INVENTION

The process of the present invention has been attained by comparison andstudy of those prior art processes quantitatively in detail, analyzingthe steps of separating excess ammonia and decomposing/separatingammonium carbamate and returning those products into the reaction zone,individually and comprehensively in connection with the properties ofthe treated substances and investigating reationalization of the stepsof urea production. It has been found, according to the process of thepresent invention, that the efficiency of the decomposition andseparation can be improved unexpectedly as compared with those obtainedby known processes wherein unconverted ammonium carbamate is decomposedand thereby separated under a high pressure.

If the technique of separating the gas formed by the decomposition fromthe liquid phase of the effluent from the reactor under a high pressureis improved, the decomposition is also accelerated consequently andfavorable results can be obtained. For reducing the loss of power usedfor maintaining the pressure required in the synthesis zone and also forutilizing most of the high energy level of the process flow, ammoniumcarbamate is decomposed and thereby separated by introducing theeffluent from the synthesis zone into an ammonium carbamatedecomposition/separation zone without effecting pressure reduction. Foraccelerating the separation of the decomposed gas from the liquid phaseunder the high pressure, the free surface of the liquid effluent isenlarged in the process of the present invention. Residence timesufficient for the decomposition and separation is provided. In a stagein which the concentration of the unconverted substances is high and inwhich decomposition and separation are relatively large in amount, thedecomposition and separation are effected by supplying only heat foreffecting ammonium carbamate decomposition or, alternatively, by usingstarting ammonia as a stripping gas and the gaseous mixture thusseparated out is removed therefrom so as to prevent re-dissolution ofthe gaseous mixture separated from the liquid phase and re-formation ofammonium carbamate. In a stage in which the concentration of theunconverted substances is low and in which decomposition and separationare relatively small in amount, ammonium carbamate remaining in the ureasolution is reduced in amount by employing both supply of heat forammonium carbamate decomposition and stripping with starting carbondioxide.

The combination of the above described means brings about a multipliedeffect.

The absence of the gas decomposed and thereby separated from thesubsequent step in the stage in which the gaseous mixture formed bydecomposition and separation is large in amount is advantageous for thedecomposition/separation of ammonium carbamate and for the separation ofexcess ammonia.

By providing the decomposition/separation treatment in the first step,load in the step the of the decomposition/separation in which carbondioxide is used as the stripping gas becomes low, both the decompositionand separation in this step become effective and the remaining ammoniumcarbamate is reduced in amount, whereby the load in the subsequent stepof purifying the urea solution under reduced pressure is reducedremarkably.

The present invention provides a process for synthesizing urea byreacting ammonia with carbon dioxide, as starting materials, in areactor, at a high temperature, under a high pressure, to obtain ureaand unconverted materials, decomposing the unconverted materials,separating the decomposition products from urea, i.e. the intendedproduct, and returning them into the reactor, wherein the effluentcontaining urea, ammonium carbamate, excess ammonia and water from thereactor is introduced in a first heater to decompose and thereby toseparate the excess ammonia and ammonium carbamate from the liquid phaseunder substantially the same pressure as in the reactor; then the liquideffluent from the first heater is introduced in a second heater toseparate remaining excess ammonia and also to decompose ammoniumcarbamate in the presence of starting carbon dioxide which acts as astripping gas, thereby separating the same from the liquid phase undersubstantially the same pressure as in the reactor; the liquid effluentfrom the second heater is sent into a step of purifying the ureasolution under a low pressure; and the gaseous, separated effluentsdischarged from the first and the second heaters are returned into thereactor.

As in the first and second heaters, there are used heat exchangers ofthe falling thin film type in which the separation of ammonia anddecomposition of ammonium carbamate are accelerated. The gaseouseffluents from the first and the second heaters are sent into thereactor together with the solution returned from the urea purificationstep operated under low pressure. The ratio of the amount of theunconverted substances which are decomposed and/or separated in thefirst and the second heaters and distilled from them is 30-80%,preferably 50-70%, in the first heater and 20-70%, preferably 30-50%, inthe second heater.

It is important in the process of the present invention thatmultiplication of decomposition and separation of the unconvertedcomponents under high pressure is realized by providing separation ofgaseous effluent from the process flow between the first heater and thesecond heater. Therefore, a third heater may be used to provide theseparation of gaseous effluent not in one step but in two steps, if thecost of the equipment makes that permissible. It is important tominimize outflow of substances other than the aqueous urea solution fromthe high pressure system under substantially the same pressure as in theurea synthesis, which comprises the reactor, first and second heatersand a mixer for collecting the gaseous effluents from those heaters.

In oder to minimize the amount of outflow of said other substances fromthe high pressure system, plural heaters are provided, gaseous effluentsfrom those heaters are returned to the reactor and unconvertedsubstances are selectively retained in the high pressure system.

Except for the aqueous urea solution, the unconverted substances are nottransferred to a low energy state and, therefore, a remarkable effectcan be obtained.

In the accompanying FIG. 1 flow sheet, the invention is illustrated asan example, which is given by way of illustration and not of limitation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet showing steps of the process of the presentinvention.

FIG. 2 is a flow sheet showing steps of a process to be compared withthe process of the present invention.

DETAILED DESCRIPTION

The process of the present invention will be illustrated with referenceto FIG. 1.

Pressurized starting ammonia is introduced in a reactor 2 for ureasynthesis through a pipe 1.

It is also possible to introduce the starting ammonia, not directly intothe reactor 2, but rather, first into a first heater 3 to use it as astripping gas therein and then to introduce the same into the reactorthrough a mixer 4.

Starting carbon dioxide is used as the stripping gas supplied in a lowerpart of a second heater 6 through a pipe 5 and then introduced into thereactor 2 through the pipe 14, the mixer 4, and the pipe 8.

A recycle liquid obtained by separating and thereby recovering ammoniumcarbamate, etc. remaining in the aqueous urea solution under a pressurelower than the urea synthesis pressure is also introduced in the reactor2 through a pipe 7.

The recycle liquid of pipe 7 flows into the injector 23, together withthe high pressure returning flow of the unconverted substances suppliedthrough a pipe 8 from the mixer 4, and thence flows into the reactor 2.

A part of the recycle liquid may flow through the mixer 4 as a sideflow.

Urea is formed in the reactor 2 at a high temperature under a highpressure, in the presence of excess ammonia (i.e. at a molar ratio ofNH₃ to CO₂ of, for example, 4:1). Urea is introduced together withunconverted ammonium carbamate, excess ammonia and water, as an effluentfrom the reactor 2, into the first heater 3 through a pipe 9 and theninto a second heater 6 through a pipe 10. The unconverted ammoniumcarbamate and excess ammonia in the effluent are separated as a gaseousmixture from the liquid phase flow which is in the form of a fallingthin film under substantially the same pressure as in the ureasynthesis.

In the second heater 6, the starting carbon dioxide is utilized as thestripping gas for accelerating the separation of the gaseous mixture asdescribed above. Into the first and the second heaters, steam forheating is supplied through pipe 11 as needed.

In the mixer 4, ammonia reacts with carbon dioxide vigorously togenerate a large amount of heat of reaction. The heat of reaction isrecovered as steam through a pipe 12.

The thus-recovered heat of reaction generated at a high energy level inthe mixer 4 can be used, if desired, as a direct heat source in anaqueous urea solution purification step under low pressure, for example,in a heating device 18 to exhibit a higher efficiency.

A description will be made on the first and second heaters which areimportant parts in the process of the present invention. In the heater3, the effluent from the reactor 2 flows down in the form of thin filmson the inner surfaces of numerous vertical tubes. By the heat of theheating media supplied as needed on the outer surfaces of the verticaltubes, the remaining ammonium carbamate unconverted into urea isdecomposed into ammonia and carbon dioxide, which are separated from theliquid effluent, together with excess ammonia and these gases areintroduced into the mixer 4 through a pipe 13.

As the vertical tubes in the heater 3, cylindrical tubes or doublefluted tubes may be used. The separated gaseous mixture flows incountercurrent flow with respect to the liquid effluent flowingdownward, as is shown by the position of pipe 13 in FIG. 1 or,alternatively, it may flow concurrently with the liquid effluent bychanging the position of pipe 13.

Either a countercurrent or a concurrent flow can be selected easily,since the separated gaseous mixture is taken out, not just once, buttwice, through pipes 13 and 14 in the decomposition/separation steps,under substantially the same pressure as in the reactor and can not stayin a large amount locally in the heaters.

As heaters are used at the two decomposition/separation steps, thedifference in quantity of liquid between the inlet and outlet of eachheating tube is small and a stable falling thin film is thus maintainedfavorably.

It is an advantage of the present invention that either a countercurrentflow by which the required residence time is secured at a lowflowing-down velocity or a concurrent flow by which high rates of massand heat transfer are attained at a high flowing-down velocity may beselected freely.

By virtue of those merits, the unconverted substances can be retainedhighly selectively in the high pressure system.

The effluent from the reactor which reaches the bottom of the heater 3is supplied into an upper part of the second heater 6 through the pipe10 and flows downwards in the form of thin films on the inner surfacesof the numerous vertical tubes. Ammonium carbamate is further decomposedby heat supplied by the heating media fed as needed onto the outersurfaces of the vertical tubes. It is subjected to stripping by thestarting carbon dioxide fed through the pipe 5. The gaseous mixtureexpelled from the effluent from the reactor in the heater 6 is sent intothe reactor 2 through a pipe 14 via the mixer 4.

Thus, the substances remaining unconverted into urea, including theexcess ammonia, are prevented from outflowing from the high pressuresystem and the retained therein. Only the aqueous urea solution flowsout selectively.

If the entire high pressure system is regarded as the reactor for ureasynthesis, this reactor has internally a mechanism of recycling theunconverted substances as starting materials and produces urea at aconversion higher than the quilibrium conversion.

The effluent from the reactor then reaches the bottom of the heater 6.Thereafter, it is sent to a pressure reducing valve 16 through a pipe 15and the pressure is reduced to lower than the urea synthesis pressure.It is then introduced in a decomposition device 17, in which theremaining unconverted substances are removed by decomposition with aheating media supplied through a pipe 18 under the pressure lower thanthe urea synthesis pressure, thereby purifying the aqueous ureasolution.

The substances removed from the solution in the decomposition device 17are introduced into an absorption device 20 through a pipe 19 anddissolved in an absorbing liquid supplied through a pipe 21. The liquidthus formed is then pumped with a pump 22 and is introduced into aninjector 23 through a pipe 7. The liquid merges the effluent from themixer 4 and flows into the reactor 2. The aqueous urea solution purifiedin the decomposition device 17 is sent into the subsequent purificationstep (not shown in the figure) through a pipe 24.

For comparison with said process of the present invention, anotherprocess is illustrated in FIG. 2.

This process consistutes a different process comprising the combinationof two known processes.

As compared with a case of employing only one known process, utilitycost of urea production can be improved. But, as will be easilyunderstood from the construction of this process, the effect ofmultiplication achieved by the process of the present invention cannotbe obtained thereby.

A difference between FIGS. 1 and 2 is that heaters 3 and 6 in FIG. 2 arepositioned in parallel to share the load.

Effects achieved by the process of the present invention are shown belowas compared with those of a conventional process:

    ______________________________________                                        Urea production (t/day)  1,000                                                Starting ammonia (t/day) 566                                                  Starting carbon dioxide (t/day)                                                                        735                                                  Pressure in reactor 2 (Kg/cm.sup.2 G)                                                                  200                                                  Temperature in reactor 2 (°C.)                                                                  191                                                  Pressure in decomposition device 17                                           (Kg/cm.sup.2 G)          17                                                   Temperature of fluid in pipe 24 (°C.)                                                           165                                                  Heating steam consumption                                                                        Conventional                                                                             Process of                                                         process    the present                                                                   invention                                       25 Kg/cm G Steam (t/t urea)                                                   (for the first and the second                                                 heaters)           --         0.66                                            12 Kg/cm G Steam (t/t urea)                                                   (for the decomposition device)                                                                   0.63       --                                                                 Conventional                                                                             Process of                                                         process    the present                                                                   invention                                       (for others)       0.32       0.02                                            Subtotal (t/t urea)                                                                              0.95       0.68                                            5 Kg/cm G recovered                                                           steam (t/t urea)   --         0.37                                            Total (t/t urea)   0.95       0.31                                            Cooling water (t/t urea)                                                                         85         50                                              Electric power (KWH/t urea)                                                                      150        142                                             ______________________________________                                    

We claim:
 1. In a urea synthesis process in which ammonia and carbondioxide are reacted in a reactor, at an elevated temperature andpressure, to form a urea synthesis effluent stream containing urea,ammonium carbamate, ammonia and water, the improvement which comprises:passing said effluent stream downwardly through a first heating zonewhich is maintained at substantially the same pressure as said reactorand which is externally heated so as to decompose a large amount of saidammonium carbamate to form ammonia and carbon dioxide and to distill offammonia and carbon dioxide gases; separating the effluent stream in saidfirst heating zone into a first gas phase containing ammonia and carbondioxide and a first liquid phase containing said urea, said water andreduced amounts of ammonium carbamate and ammonia, then passing saidfirst liquid phase downwardly through a second heating zone which ismaintained at substantially the same pressure as said reactor and whichis externally heated so as to decompose ammonium carbamate in said firstliquid phase to form ammonia and carbon dioxide and to distill offammonia and carbon dioxide gases, and simultaneously contacting saidfirst liquid phase with CO₂ stripping gas fed into said second heatingzone from an external source thereof whereby to remove ammonia from saidfirst liquid phase; separating said first liquid phase in said secondheating zone into a second gas phase containing ammonia and carbondioxide and a second liquid phase of an aqueous urea solution; thenreducing the pressure of said second liquid phase and feeding samethrough a urea purification stage which is at a lower pressure than saidreactor whereby to obtain a purified aqueous urea solution; andcontinuously maintaining said first gas phase and said second gas phasesubstantially at a pressure of said reactor and feeding said first gasphase and said second gas phase into said reactor.
 2. A urea synthesisprocess as claimed in claim 1 in which said first heating zone andsecond heating zone each comprise a plurality of externally heatedvertical tubes and wherein said effluent stream flows downwardly in theform of thin films on the internal surfaces of said vertical tubes ofsaid first heating zone and said first liquid phase flows downwardly inthe form of thin films on the inernal surfaces of said vertical tubes ofsaid second heating zone.
 3. A urea synthesis process as claimed inclaim 2 in which said first gas phase is removed from the upper end ofsaid first heating zone, said first liquid phase is removed from thelower end of said heating zone, said second gas phase is removed fromthe upper end of said second heating zone and said second liquid phaseis removed from the lower end of said second heating zone.
 4. A ureasynthesis process as claimed in claim 2 in which said first gas phaseand said first liquid phase are separately removed from the lower end ofsaid first heating zone, and said second gas phase and said secondliquid phase are separately removed from the lower end of said secondheating zone.
 5. A urea synthesis process as claimed in claim 1 or claim2 in which the conversion of ammonium carbamate to urea in said reactoris from 50 to 75 percent and wherein from 30 to 80 percent of saidammonium carbamate in said effluent stream is decomposed and separatedin said first heating zone and from 20 to 70 percent of said ammoniumcarbamate in said effluent stream is decomposed and separated in saidsecond heating zone.
 6. A urea synthesis process as claimed in claim 1or claim 2 in which the conversion of ammonium carbamate to urea is from50 to 75 percent and wherein from 50 to 70 percent of said ammoniumcarbamate in said effluent stream is decomposed and separated in saidfirst heating zone and from 30 to 50 percent of said ammonium carbamatein said effluent stream is decomposed and separated in said secondheating zone.
 7. A urea synthesis process as claimed in claim 1 or claim2 in which in said first heating zone, said effluent stream is contactedwith a stream of ammonia gas effective to strip said gaseous ammonia andgaseous carbon dioxide formed by decomposing said ammonium carbamate. 8.A urea synthesis process as claimed in claim 1 in which in said ureapurification stage, said second liquid phase is heated to decompose anyremaining ammonium carbamate present therein, removing a third gas phasefrom said urea purification stage and absorbing said third gas phase inan absorbing liquid.
 9. A urea synthesis process as claimed in claim 1or claim 2 in which said first and second gas phases are mixed togetherin a mixing vessel and then the resultant mixture thereof is injectedinto said reactor.
 10. A urea synthesis process as claimed in claim 9 inwhich in said urea purification stage, said second liquid phase isheated to decompose any remaining ammonium carbamate present therein,removing a third gas phase from said urea purification stage andabsorbing said third gas phase in an absorbing liquid, and mixing saidabsorbing liquid with said first and second gas phases in said mixingvessel.